The voltage source converter based high voltage direct current (VSC-HVDC) uses a power electronic device that can be turned-off, and it can commutated without the need for an external power supply, thus providing the ability to supply power to a passive grid. In the operation of the power grid, in order to enhance the reliability of power supply for important loads, two-way power supply, i.e., flexible DC and AC lines, can be used to supply power to the important loads. Under normal circumstances, the two-way are used to supply power. In special cases, the power is supplied by a flexible DC or AC line. When the flexible DC power supply mode is adopted, in the case of a failure on the AC side, the AC protection trips the faulty line to remove the fault, and the flexible DC system continues to operate. However, in special cases, the flexible DC system does not coordinate with the AC protection, and there is no current limiting measure in the case of flexible DC islanding operation, which may cause that the flexible DC first trips out in the case of a fault outside the flexible DC system, the AC protection does not act or delays the action, and the flexible DC converter instantaneous overload and output current limiting capability are not fully utilized to form the coordination between AC and DC protection. AC protection generally includes overcurrent protection section I, II or III. The flexible DC converter has bridge arm overcurrent protection section I, II or III. By setting coordination between the AC and DC overcurrent protection with set values, in the case of a fault outside the converter, the current limiting capability of the converter can be utilized to achieve a given output current level in the event of the fault, and a partial short-circuit current is provided. In this case, the AC protection can be used to remove the fault, and on the other hand, the converter operation is not affected in the case of the fault.
According to a patent entitled “A Smooth Switching Method for Flexible DC Transmission System from Parallel Operation to Islanding Operation”, Chinese application number 201310220297.8, detecting flexible DC transmission system operated in AC/DC parallel operation mode or islanding operation mode by system electrical quantity change characteristics, and after determining that the system is working in islanding operation mode, the control mode is automatically switched to the corresponding control strategy, but there is no description of the coordination with AC protection in the case of a fault in islanding operation mode. “MMC-HVDC Networking and Islanding Operation State Transition Strategy”, 2015, 35 (9): 2152-2161, describes a stable switching strategy between a networked state and an islanding state of the flexible DC system, but does not involve a control current limiting method and AC system protection coordination mode under the protection fault.